In recent years, carbon-neutral biodiesel fuel has been gaining attention as a diesel engine fuel. A known method for obtaining biodiesel comprises: carrying out a transesterification reaction of monohydric alcohol and triglyceride contained in a raw material such as a plant oil, animal oil, or waste plant or animal oil; and removing glycerin formed as a by-product (Chemical formula 1). In the reaction of Chemical formula 1, biodiesel is obtained in the form of an ester of low viscous monohydric alcohol and fatty acid. Such ester can be used as a fuel.
Glycerin obtained as a by-product upon biodiesel production also contains a catalyst, unesterified fatty acid, and the like. Such glycerin cannot be effectively used and thus it is discarded under present circumstances. Therefore, effective use of glycerin obtained as a by-product is a recent object in the art.

In general, glycerin is used as a raw material for producing nitroglycerin, pharmaceutical products, cosmetics, sweeteners, and the like. However, in any case, high-purity glycerin is necessary, while on the other hand, there is no large demand for glycerin production. For such reasons, it is impossible to sell glycerin purified from glycerin obtained as a by-product upon biodiesel production given the prices in view of the purification cost and the amount of purified glycerin under present circumstances.
Patent Document 1 describes a biodiesel production method wherein glycerin obtained as a by-product is purified via distillation or the like. However, the method described in Patent Document 1 comprises a step of purifying glycerin, resulting in additional cost increase. In this regard, it is difficult to achieve excellent economic efficiency. Further, it is not always possible to obtain high-purity glycerin with the method. Therefore, it would be highly probable that the method would be inappropriate for commercial use.
Patent Document 2 describes a method for producing biodiesel and glycerin using high-temperature methanol at about its critical temperature without an alkali catalyst. In this method, there is no need to use an alkali or a neutralizer that becomes an impurity, facilitating purification of biodiesel and glycerin. However, this production method requires much greater energy than the alkali catalyst method. The method is too costly to produce less expensive materials, which is problematic.
In addition to the production methods described in Patent Documents 1 and 2, there are known methods for obtaining high-purity glycerin via an enzyme method, a supercritical method, and the like. However, all of the above production methods cause cost increase, making it difficult to achieve excellent economic efficiency. Therefore, it is difficult to use such methods for industrial applications.
Meanwhile, when glycerin is used as a chemical, it is used as a raw material for producing acrolein, which is a raw material for acrylic acid, 1,3-propanediol, or the like.
Non-Patent Document 1 describes a method for producing acrolein by adding sulfuric acid to glycerin and treating the resultant with supercritical water. According to this method, acrolein can be obtained from glycerin at a yield of 70% or more because of the high catalyst activity of protons in supercritical water.

In Non-Patent Document 1, the method is merely examined under conditions comprising a glycerin concentration in a reaction mixture subjected to a supercritical reaction of as low as approximately 1.0%. However, for industrial applications, highly concentrated glycerin must be used as a raw material in consideration of efficient use of energy for preparation of supercritical water.
Non-Patent Document 2 describes a method for synthesizing acrolein from glycerin via treatment with an acid and supercritical water. According to the method, in order to maintain the raw material yield at a high level for acrolein synthesis even at a high glycerin concentration, it is necessary to set the reaction time and the catalyst aid (acid) concentration at levels appropriate for high glycerin concentrations. According to Non-Patent Document 2, it is desirable that the relation between the glycerin concentration in a reaction mixture subjected to a supercritical reaction ([G]R) and the hydrogen ion concentration ([H+]R) expressed by the following equation be satisfied: [H+]R2/[G]R=15 to 25 mM2/wt % (and particularly desirably 18 to 22 mM2/wt %). If the above relation between the glycerin concentration and the hydrogen ion concentration does not fall within the above ranges (that is to say, if the acid concentration is excessively low or high), by-products such as tar and carbon are increasingly produced, resulting in reduction of raw material yield. Further, such by-products might cause a risk of obstruction in a reactor or pipes. This might interrupt stable plant operation.